1. Field of the Invention
This invention relates to a method and apparatus for detecting positions of elements. Specifically it relates to a position detecting method and a position detecting apparatus for light emitting elements used, for example, in an image forming apparatus which forms an image by correcting the light emitting amount from each light emitting element of a print head array.
2. Discussion of the Related Art
In recent years, a print head array where micro light emitting elements are arranged in a predetermined pitch has been used in an image forming apparatus that rapidly forms a highly resolved image. However, the light amount emitted from each light emitting element generally varies. The variation in the light amount emitted is recorded just as are irregularities in the light and shade amount of the image that definitely cause a deterioration of picture quality. Therefore correction of the luminous power is performed so that the light amount emitted from each light emitting element is uniform. For correcting the luminous power, it is necessary to detect the luminous power of every light emitting element.
Previously the light amount emitted was detected by making a light receiving element scan along an imaging face of a plurality of light emitting elements arranged in one direction with a pulse motor. It had been accomplished by correlating a number of pulses given to the pulse motor and the position of light emitting element. Therefore each light emitting element position was virtually detected based on a counted value of the pulse number. The light receiving amount at that position was recognized as a light emitted amount corresponding to that light emitting element.
However in correlating the number of pulses and the light emitting element positions, errors could not be avoided and it was difficult to detect the light emitting element positions based on the counted value of the pulse number. Therefore, if the plurality of light emitting elements arranged in one direction had a short length, the required precision could be obtained. However when a longer length was involved, the difference caused by errors became to large to be ignored. Thus the difference between the light emitting element position virtually detected based on the number of pulses and the actual light emitting element position occurring resulted in the proper position of the light emitting element not being detected.
Further, to correlate the counted value of the pulse number and the position of the light emitting element with as small an error as possible, it was necessary to accurately establish a base position of the light receiving element as well as an amount of movement of the light emitting element from the base position. This then required very high precision in machining and assembling the apparatus and it deteriorated the ease of operation.
Therefore, the primary objects of this invention are to provide method and apparatus for detecting positions which can accurately detect the position of the light emitting elements and improve the ease of operation.
To solve the above problems, in one aspect of this invention, a position detecting method for detecting a position of each light emitting element of a print head having a plurality of light emitting elements arranged along one direction is disclosed. The position detecting method comprises lighting selected light emitting elements among the plurality of light emitting elements, measuring luminous power emitted from the light emitting elements by making a light receiver scan in the direction of arrangement of the light emitting elements, obtaining a data series indicating luminous power based on the results of said step of measuring, comparing the data series with a threshold and detecting a first place that exceeds the threshold from the beginning of the data series, determining a position of a maximum value closest to the first place as a first light emitting element position among the lighted light emitting elements, and determining additional positions of maximum value as light emitting element positions in terms of the positions of the light emitting elements after the first light emitting element position.
In a second aspect of the invention, the step of lighting selected light emitting elements further comprises alternately lighting the plurality of light emitting elements.
In a third aspect of the invention, the step of determining a position of maximum value further comprises expanding a searching range and detecting the maximum value when the maximum value can not be detected within a searching range of one cycle.
In a fourth aspect of the invention, the step of determining additional positions of maximum value further comprises detecting the maximum value within a searching range of one cycle after a half cycle following the position of a preceding light emitting element after the first light emitting element position is detected.
In a fifth aspect of the invention, the step of determining additional positions of maximum value further comprises treating a center of the searching range of one cycle as the light emitting element position when the maximum value is detected farther away from a center of the searching range of one cycle than a predetermined position.
In a sixth aspect of the invention, the threshold is any one of a plurality of minimum values contained in the data series from the light receiver.
In a seventh aspect of the invention, the threshold is an arbitrary value.
In an eighth aspect of the invention, the print head includes a lead lanthanum zirconate titanate (PLZT) shutter array.
In a ninth particular aspect of the invention, a position detecting apparatus for detecting a position of each light emitting element of a print head having a plurality of light emitting elements arranged in one direction is disclosed. The position detecting apparatus comprises lighting means for selectively lighting the light emitting elements, light receiving means for measuring luminous power emitted from said light emitting elements, scanning means for making said light receiving means scan in the direction said light emitting elements, means for obtaining at least one data series indicating luminous power based on measurements obtained by said light receiving means, detecting means for comparing the data series with a threshold and detecting a first place exceeding the threshold from the beginning of the data series, means for determining a position of maximum value closest to the first place as a first light emitting element position among the light emitting elements lighted by said lighting means, and means for determining additional positions of maximum value as light emitting element positions in terms of the positions of the light emitting element after the first light emitting element position.
According to one of the aspects of the invention, the amount of light received is measured by scanning along the imaging face of the plurality of light emitting elements arranged in one direction and based on these measurements, the predetermined cycle of data series are obtained. Then, a first place exceeding the threshold from the beginning of the data series is detected, the maximum value closest to that place is detected, and the position of the maximum value is considered as the beginning or first light emitting element position. Thereafter, the positions of the maximum values are considered as the light emitting element positions and a similar process is repeated to detect the light emitting element positions until the last light emitting element position is detected.
Herein, among the imaging faces, the amount of light received from a position where the light from each light emitting element is perpendicularly irradiated is larger than amount of light received from its surrounding portion. Thus the data series indicating the luminous power has a maximum amount at the perpendicularly irradiated position. This position having the maximum value corresponds to the position of light emitting element. Accordingly the detecting of the maximum value of the data series allows the detection of the position of light emitting element. Then even in the case of a long length display, errors are unlikely to occur and the position of the light emitting elements can be accurately detected. Further the position detection is performed by detecting the maximum value, so high precision is not required for the mechanical machining and assembly.
These features are similar with regard to the position detection apparatus according to the ninth particular aspect of the invention.
In the case where the spacing between the light emitting elements is very small and the data series from which a maximum value can be detected is not obtained, the data series from which the maximum value can be detected becomes obtainable by alternately lighting the light emitting elements as the position detecting method according to the second aspect of the invention. Then based on this data series, the position of light emitting element can be detected.
According to the third aspect of the invention, when the maximum value can not be detected within a searching range of one cycle from the first place, the maximum value is detected by expanding the searching range. Accordingly, when the maximum value within the searching range lies at the end of the searching range, misdetection of the maximum value by considering the maximum amount as the end point of the range is eliminated and the precision for detecting the positions of the light emitting elements can be improved.
According to the fourth aspect of the invention, it is realized that only one maximum value usually exists in one cycle of the data series. Then the searching range for the maximum value is set within one cycle from a half cycle after the preceding position of light emitting element.
According to the fifth aspect of the invention, taking into account that when the maximum value is detected farther away from a center of the searching range of one cycle than the predetermined position, failure of the light emitting elements, etc. is assumed to be its cause. Then in this case, this affects the detection of the following light emitting element positions, when the maximum value is detected farther away from the center of the searching range than the predetermined position. Thus the light emitting element is deemed to have failed and the center of the searching range is considered as the light emitting element position. Thereby, failure of a light emitting element can not influence the detection of the following light emitting elements position.
Further, according to the sixth or seventh aspect of invention, the threshold can be any one of a plurality of minimum amounts contained in the data series or even an arbitrary value. According to the eighth aspect of the invention, the print head can be, for example, a PLZT shutter array.
According to this invention, it is taken into account that the data series indicating the luminous power at the position where the light from each light emitting element is perpendicularly irradiated among the imaging faces of the light emitting elements of the print head is maximum and that the position with the maximum value corresponding to the light emitting element position is detected by detecting the maximum value of the data series so the light emitting element position is detected. Accordingly in the case of the longer length display, errors are unlikely to occur, the light emitting element can be accurately detected and the proper luminous power correction becomes possible.
Further, the light emitting element position is detected by detecting the maximum value, different from the prior method for detecting the light emitting element position based on the pulse number of the motor and therefore, high precision for the mechanical machining and assembly is not required, and the ease in operation is improved.